One link of an optical telecommunication system typically has a transmitter, an optical fiber, and a receiver. The optical transmitter includes a modulated laser source, which converts an electrical signal into a modulated optical signal and launches it into the optical fiber. The optical fiber transports the optical signal to the receiver. The receiver converts the modulated optical signal back into an electrical signal. Typically the electrical signal takes the form of an RF electrical signal that encodes information to be transmitted; the optical signal is modulated by the RF signal to carry the information along an optical fiber.
An optical transmitter for the transmission of an analog or digital radio-frequency (RF) signal over an optical fiber can use a directly modulated laser or a continuous wave (CW) laser coupled to an external modulator. Directly modulating the analog intensity of a light-emitting diode (LED) or semiconductor laser with an electrical signal is among the simplest methods for transmitting analog signals, such as voice and video signals, over optical fibers. Although such analog transmission techniques have the advantage of substantially smaller bandwidth requirements than digital transmission, such as digital pulse code modulation, or analog or pulse frequency modulation, the use of amplitude modulation typically places more stringent requirements on the noise and distortion characteristics of the transmitter. A limiting factor in such links can be the second order distortion due to the combination of optical frequency modulation (i.e., chirp) and optical fiber dispersion.
For these reasons, direct modulation techniques have typically been used in connection with 1310 nm lasers used for relatively short transmission links that employ optical fiber with relatively low dispersion. It is also possible to use direct modulation of 1550 nm lasers, but in this case the distortion produced by chirp and dispersion typically must be cancelled using a programmable predistorter that is set for the specific fiber length. In some case, such as when the signal must be sent to more than one location or through redundant fiber links of different lengths, such a predistorter can be undesirable. To avoid the distortion problems related to chirp and dispersion at 1550 nm with direct modulation, low chirp external optical modulators are commonly used in analog fiber optic communication systems, such as CATV signal distribution, to amplitude modulate an optical carrier with an information or content-containing signal, such as audio, video, or data signals.
Since the present disclosure also relates to external optical modulators associated with a laser, a brief background on external optical modulators is noted here. There are two general types of external optical modulators implemented as semiconductor devices known in the prior art: Mach Zehnder (MZ) modulators and electro-absorption (EA) modulators. A Mach-Zehnder modulator splits the optical beam into two arms or paths on the semiconductor device, one arm of which incorporates a phase modulator. The beams are then recombined which results in interference of the two wavefronts, thereby amplitude modulating the resulting light beam as a function of the modulated bias signal applied to the phase modulated arm. An electro-absorption modulator is implemented as a waveguide in a semiconductor device in which the absorption spectrum in the waveguide is modulated by an applied electric bias field, which changes the band gap energy in that region of the semiconductor, thereby modulating the amplitude or intensity of the light beam traversing the waveguide.
Stimulated Brillouin scattering (SBS) effects that depend on the optical launch power and the total fiber length may also degrade fiber optic system performance. SBS is an opto-acoustic nonlinear process that can occur in single mode optical fibers. This optically induced acoustic resonance effectively limits the amount of optical power that can be successfully transmitted through the single mode optical fiber within a given bandwidth.
The SBS can perhaps be best explained in terms of three waves in an optical fiber. When an incident wave (also known as the “pump wave”) propagating along the optical fiber reaches a threshold power (which may vary), it excites an acoustic wave in the optical fiber. The optical properties of the optical fiber such as the refractive index are altered by the acoustic wave, and the fluctuation in the refractive index scatters the incident wave, thereby generating a reflected wave (also known as the “Stokes wave”) that propagates in the opposite direction.
Because of the scattering, power is transferred from the incident wave to the reflected wave, and molecular vibrations in the optical fiber absorb the lost energy; the reflected wave has a lower optical frequency than the incident wave. The scattering effect can result in attenuation, power saturation and/or backward-propagation, each of which deteriorates the DWDM system performance. The attenuation is caused by the transfer of power from the incident wave to the acoustic and reflected waves; due to power saturation, there is a limit to the maximum amount of power that can be transmitted over the optical fiber; the backward propagation wave can create noise in transmitters and saturate amplifiers.
The phenomenon of SBS has been known by optical network equipment designers for a number of years. SBS results when a threshold power level is exceeded within a sufficiently narrow frequency band in a fiber optic light guide. The increasing operational relevance of SBS relates to the development of lasers such as, for example, single longitudinal mode lasers which can readily provide an output that exceeds the SBS threshold (e.g., typically about 4 mW within an optical bandwidth of about 25 MHz over about 50 km of single-mode optical fiber). Moreover, limitation of optical power to a level as low as 4 mW not only fails to utilize the output power available from state of the art lasers, but limits distance transmission through fiber optic cable by an unacceptable margin.
Various approaches to minimize the effect of SBS are also known. In general, SBS impact can be reduced in an externally modulated analog system if the optical signal's spectrum can be broadened, thereby lowering optical power per unit bandwidth. Some effective and widely used techniques for combating SBS include the use of an optical phase modulator or dithering the laser frequency or both, in the case of externally modulated laser sources.